Rework istio/k8s config to use the new Gateway API

[Istio has beta support](https://istio.io/latest/news/releases/1.16.x/announcing-1.16/#kubernetes-gateway-api-implementation-promoted-to-beta) for the new [Gateway API](https://gateway-api.sigs.k8s.io/). This looks to be a really human/developer friendly way of configuring Istio and we should try it out.